The method and system to which the invention relates, may be used for example as part of a method or system for analysing the perceived quality of an audio transmission system. Such method and system for analysing a perceptual quality measure for the impact of linear frequency distortion are known from a previously published European patent application no EP1343145 and are also disclosed in references [1] . . . [8]. The disclosed system and method and its predecessors provide for perceptual speech evaluation as part of ITU-T recommendation P.862 (further referred to as P.862), whereby a single overall measure for the perceived quality of a degraded output signal with respect to an input signal is obtained.
The disclosed method and system are based on the insight that speech and audio quality measurement should be carried out in the perceptual domain (see FIG. 1). This goal is achieved by comparing a reference speech signal Xn, that is applied to the system under test (1), with its degraded output signal Yn. By establishing the internal perceptual representations of these signals (0.1), (0.2) and comparing (0.3) them, an estimate can be made about the perceived quality by mapping (0.4) the result to a perceived quality scale, yielding a perceived quality measure PESQ. A perceived quality scale, also known as a mean opinion scale (MOS) is established in the prior art by empirical estimation. Persons are asked to judge the quality of degraded or distorted speech fragments. The scores are then matched to the actual distortion and laid down in a perceptual scale. This scale can be used to predict the perceptual score depending on the distortion present in a signal.
Currently available processing systems for determining perceived quality of an audio transmission system, including P.862, suffer from the fact that a single number is outputted that represents the overall quality. This makes it impossible to find underlying causes for the perceived degradations. Classical measurements like signal to noise ratio, frequency response distortion, total harmonic distortion, etc. pre-suppose a certain type of degradation and then quantify this by performing a certain type of quality measurement. This classical approach finds one or more underlying causes for bad performance of the system under test but is not able to quantify the impact of the linear frequency response distortion in relation to the other types of distortion with regard to the overall perceived quality.
Furthermore, the performance of currently available methods and processing systems for determining perceived quality of an audio transmission system, including P.862, give inadequate results, since the perceived linear frequency distortion is not treated properly in those systems.
The above methods utilise frequency compensation of an input power density function, derived from the input signal, for the purpose of quantifying the effect that linear frequency response distortions have less impact on the perceived speech quality than non-linear distortions.
The known method of frequency compensation fails because they either use a hard clipping function or a modified clipping function that do not allow to quantify the impact of linear frequency response distortions on the perceived speech quality in a perceptual correct manner.